Although a famous species, H. crassum is actually poorly known : there are no complete skulls, only scanty limb bones and a few teeth that may, or may not, belong to a single species.
A cranium of H. crassum mentioned by Depéret (1890) and chosen as lectotype by Forsten (1968) was never described in detail and is apparently lost. There are other fossils (particularly in Basel) that I had no opportunity to see.
Several fragmentary mandibles belonging to the collections of the Muséum d’Histoire Naturelle de Lyon could be found and restored. There are two fragmentary probably adult symphyses (both labelled Pp 215), and four more or less complete mandible (Pp 206, 208, 209, and 210). Specimens Pp 206, 209, and 210 belong to immature animals ; their snouts were wrongly reconstructed including not belonging incisors insite the plaster. Specimen Pp 208, although fragmentary is more intersting : it belongs to a nearly adult animal and the snout is well preserved.
There are a dozen of MC and MT each, including many juvenile and fragmentary specimens and a few other bones. They are all supposed to come from Perpignan except three tibiae (Montpellier, and a badly preserved first phalanx (La Pompignanne).
There is no information about the occurrence of a preorbital fossa. The only measurements given by Depéret are the distance between the Prosthion and the posterior border of M3 (325mm) and the length of the upper and lower cheek rows (respectively 170 and 164mm). The muzzle was therefore about155mm in length, the longest recorded in hipparions.
– Upper cheek teeth
The upper cheek teeth have 10 to 38 fossette plications, multiple plis caballin, and very short and rounded protocones. The sizes at mid crown vary between 21mm (smallest M1 or M2) and 29mm (largest P3 or P4). An unworn premolar is 53mm high ; an unworn molar is 49mm high.
The extremely elongated and narrow snout points to a selective browser diet (Eisenmann, 1998). The incisors are long, straight and grooved, very much like those of African evolved hipparions (Eisenmann, 1985, pl. I). Like in most hipparions, the cups are well developed and bordered by a wavy enamel.
– Lower cheek teeth
The lower cheek teeth have quite variable enamel patterns and degrees of plication. Little worn teeth may have a nearly caballoid pattern and wrinkled fossettes. When the teeth are more worn (or cut at mid-crown), the enamel is usually (but not always) less wrinkled. Little worn premolars and molars may be up to 54mm high.
The metapodials from Perpignan are wide and flat, although not all of them to the same degree ; the length is quite variable, probably because two forms are represented. The articular facets for the fourth carpal and the fourth metacarpal are coalescent. The attachment areas of the interosseous ligaments are wide.
Of H. crassum of Perpignan, there are two probably juvenile third central phalanges. Although the retro-osseous apophyses are not developed (possibly because of the young age), the general aspect is more like in usual hipparions ; these phalanges are more stable on a horizontal plane than in H. heintzi of Calta, Turkey (Eisenmann and Sondaar, 1998). The third phalanx illustrated by Depéret (1890, pl. XIX, fig 10) looks very wide (unfortunately, we have not been able to locate the phalanges illustrated by Depéret on this plate).
As already pointed out by Forsten (1968), H. crassum is not unlike H. primigenium, and the fossils found in its company point also to humid conditions and forest environment (Aymar, 1992 ; Aguilar et al., 1998).